This invention relates to a method of moulding and to a moulding apparatus. More particularly but not exclusively the invention relates to a method of moulding a large article where tolerances are critical, such as a helicopter rotor blade which is elongate, having a substantial axial extent, or a component therefore, such as a leading edge spar.
It is known to manufacture helicopter rotor blades by draping one or usually more than one, layer of composite material in or over a male moulding tool. The composite material may typically be fibres such as carbon, glass or Kevlar fibres impregnated with an uncured resin, usually a thermosetting resin.
A membrane is then placed over the composite layers and an at least partial vacuum established between the membrane and moulding tool, to consolidate the composite material.
The method may be repeated during laying up of the layers, to consolidate and remove any air trapped between layers of the composite material. Finally, with a membrane in place, and at least partial vacuum established, the moulding tool and draped composite material are subjected to heat and pressure, typically within an autoclave, to cause or promote curing of the resin, whilst the pressure ensures consolidation of the composite material on or in the male moulding tool.
The membranes used are made in bagging materials which have only limited stretch and are unable to form to complex shapes. To overcome this problem, the membrane material may be pleated, but this can cause a pinch into which composite material may deform, which spoils the shape of the finally moulded article, and in the case of a helicopter blade, this can result in the scrapping of the moulded article.
Also, it is known to use a so-called shape intensifier, which typically, where the moulding tool is a generally male shape, is a matching female shape. The male and female shapes are relatively closed together thus preventing any thickness variations in the composite material between the shapes thus providing a definitive shape for the composite. However, particularly where the article to be moulded is a large elongate article with a substantial axial extent, such as a helicopter blade, to make a matching pair of moulding tools for the whole article can be prohibitively expensive. Accordingly it is usual for one or more shape intensifiers to be provided which are of relatively small extent compared to the main moulding tool, and are used in particularly problematical areas of the article only.
Another problem with the known method described above of moulding using bagging materials, is that by the nature of the bagging materials, generally they are usable only once, and have to be disposed of after a single use. This is because the bagging material degrades and becomes brittle due to the heat experienced in curing of the composite material, thus becoming sub-standard and a risk to re-use.
In GB 1438184 there is disclosed a method of moulding in which a rubber band is used to hold layers of composite material on a moulding tool whilst the tool and composite material layers are introduced into an elastic bag.
The moulding tool, composite material layers and the elastic bag are then placed in an autoclave where compression of the material layers is achieved by applying a vacuum.
According to a first aspect of the invention we provide a method of moulding articles from layers of composite material being fibres impregnated with uncured resin, the method including dr aping at least one layer of the composite material over a moulding tool, applying over the composite material layer or layers, a membrane, and establishing an at least partial vacuum between the membrane and the moulding tool to consolidate the composite material, characterised in that the membrane is made of a resilient material, and the method includes applying a force to the membrane to stretch the membrane over the moulding tool and maintaining the force whilst the at least partial vacuum is established.
Thus in accordance with the present invention, the membrane will conform to the shape of the moulding tool by stretching so that no pleating or the like is required to accommodate complex moulding tool shapes prior to any compression of the material layer or layers in an autoclave or the like. Furthermore, by using suitable resilient membrane materials, such as elastomeric materials, the membrane may repeatedly be re-used. Moreover, it has been found that the stretched elastomeric material acts as a shape intensifier, thus avoiding the need to provide a shape intensifier made specifically to match areas of the moulding tool.
The invention may be applied for consolidating and de-baulking individual layers, or a plurality of individual layers prior to the resin being subjected to heat or otherwise cured. However the method of the invention may include applying heat to the composite material with the membrane in a stretched condition and with the at least partial vacuum established, causing or promote curing of the resin. Thus the resin preferably is a thermosetting resin. The invention may include placing the moulding tool, uncured composite material, and stretched membrane in an autoclave in which the composite material is subjected to heat and desirably, pressure too.
According to a second aspect of the invention we provide a moulding apparatus for moulding articles from layers of composite material being fibres impregnated with uncured resin, the moulding apparatus including a moulding tool and a frame affording an opening, the opening having therein a membrane of resilient material, there being means to move the frame from one position in which the membrane is spaced from the moulding tool, to another position in which the membrane is stretched over the moulding tool with at least one layer of composite material between the moulding tool and the membrane, with there being means to maintain the membrane in a stretched condition whilst an at least partial vacuum is established between the membrane and the moulding tool.
The apparatus may further include means to apply heat to the composite material whilst the membrane is in the stretched condition and the at least partial vacuum is applied to cure or promote curing of the resin of the composite material.
The apparatus may include a base on or in which the moulding tool is provided, with there being means to provide a substantially air-tight seal between the frame and the base when the frame is in its second condition to enable the at least partial vacuum to be established between the membrane and the moulding tool.